Techniques and apparatus for improved spatial resolution for locating anomalies in optical fiber

ABSTRACT

Methods of measuring an anomaly, any induced change in physical parameters such as strain, temperature, and so forth, in an optical fiber. One method may include launching a plurality of probe pulses from a probe source; recording a Brillouin scattering spectrum from a plurality of reflection signals generated in the optical fiber, responsive to the plurality of probe pulses; determining a relative motion between the optical fiber and the anomaly during the recording the Brillouin back-scattering spectrum; and dynamically adjusting the Brillouin back-scattering spectrum according to the relative motion, or performing an adjustment of the Brillouin back-scattering spectrum after acquisition of the Brillouin back-scattering spectrum.

RELATED APPLICATIONS

The present application claims priority to U.S. provisional patentapplication Ser. No. 63/129,295, filed Dec. 22, 2020, entitledTechniques and Apparatus for Improved Spatial Resolution for LocatingAnomalies in Optical Fiber, and further claims priority to U.S.provisional patent application Ser. No. 63/059,633, filed Jul. 31, 2020,entitled Techniques and Apparatus for Improved Spatial Resolution forLocating Anomalies in Optical Fiber, each of which application isincorporated by reference herein in its entirety.

FIELD OF THE DISCLOSURE

This disclosure relates generally to the field of optical communicationnetworks and relates more particularly to techniques for distributedmeasuring moving anomalies in optical fibers. As used herein an anomalymay refer to any change in physical parameters induced in the fiber,including strain, temperature, and so forth.

BACKGROUND

Optical fibers are employed ubiquitously for applications such ascommunications in land and sea based technology. For example, opticalfibers having lengths as great as 100 km or more are commonly employedin undersea fiber optic cables. These undersea fiber optic cables arecommonly employed for transmitting data across expanses of ocean betweenterrestrial landing sites which are often located in different countriesand on different continents.

Techniques including backscattering techniques, such as BrillouinOptical Time Domain Reflectometry have been adapted for analyzingdefects or anomalies in optical fibers, either intrinsic or induced byenvironmental physical parameters around the optical fibers, wheredefects may be located at any position along many kilometers of anoptical fiber. This technique may be used to determine the location ofstrain or temperature differences in an optical fiber. This technique isnon-destructive and therefore allows for measurement of the opticalfiber at any suitable location, including at the factory, duringinstallation, or in-situ after installation of an optical cable.

Notably, a backscattering measurement may be performed as a distributedmeasurement at multiple wavelengths, to allow the acquisition ofsufficient distributed spectral property information. Duringbackscattering measurement such as when manufacturing and deployingcable, a relative motion may take place between the optical fiber usedas a sensor and the location and distribution of fiber anomalies orphysical parameters. This relative motion may accordingly skew thespatial profile of spectral properties from the backscatteringmeasurements, as required to determine the profile of physical parameterwithin the fiber, leading to degraded spatial resolution and thereforelimited accuracy.

It is with respect to these and other considerations that the presentimprovements may be useful.

BRIEF SUMMARY

A method of measuring an anomaly in an optical fiber is providedaccording to one embodiment. The method may include launching aplurality of probe pulses from a probe source into the optical fiber;recording a Brillouin back-scattering spectrum from a plurality ofreflection signals generated in the optical fiber, responsive to theplurality of probe pulses; determining a relative motion between theprobe source and the anomaly during the recording the Brillouinback-scattering spectrum; and dynamically adjusting the Brillouinscattering spectrum according to the relative motion.

In another embodiment, a method of measuring an anomaly in an opticalfiber, may include measuring a relative motion between a probe sourceand the anomaly; synchronizing a start of an acquisition of a Brillouingain spectrum (BGS) and an anomaly motion detection, wherein the BGScomprises a plurality of backscatter traces, acquired at a plurality ofinstances; and after completing of the acquisition of the BGS,correcting the BGS based on a position of the anomaly at a time when agiven BGS trace of the plurality of BGS traces is acquired.

In a further embodiment, an apparatus is provided, including a probesource; a pulse modulator to receive first portion of a probe beam fromlaser, over an optical fiber, and to output a plurality of probe pulsesto a fiber under test; and a heterodyne receiver arranged to receivesecond portion of the probe beam from the probe source, and arranged toreceive a Brillouin back-scattered portion of the probe beam from ananomaly of a fiber under test. The apparatus may further include amotion sensor or a position sensor, arranged to detect a relative motionor position, with respect to the fiber under test; and a digitalprocessor, coupled to the motion sensor or to the position sensor, fordetermining a relative motion of the optical fiber/probe source withrespect to the anomaly while measuring the Brillouin back-scatteredportion of probe beam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic diagram illustrating a conventional measurementarrangement for testing an optical fiber, according to the prior art;

FIG. 1B is a schematic diagram illustrating another conventionalmeasurement arrangement for testing an optical fiber, according to theprior art;

FIG. 2 illustrates a schematic showing components of a measurementarrangement according to the present embodiments that incorporatescomponents of the arrangements of FIG. 1A or FIG. 1B during operation;

FIG. 3A illustrates a reference Brillouin gain spectrum resulting frommeasurement of a moving anomaly in an optical fiber;

FIG. 3B illustrates a Brillouin gain spectrum resulting from measurementof the moving anomaly of FIG. 3A after correction for movement duringthe measurement or after the measurement, in accordance with embodimentsof the disclosure;

FIG. 4A illustrates a Brillouin gain spectrum resulting from measurementof a moving anomaly in an optical fiber, where Brillouin back-scatteringintensity is plotted as a function of frequency and distance;

FIG. 4B illustrates an adjusted Brillouin gain spectrum resulting fromcorrection of the Brillouin gain spectrum of FIG. 4A to account formovement of the anomaly during measurement, in accordance withembodiments of the disclosure;

FIG. 5 presents an exemplary process flow;

FIG. 6 presents an additional exemplary process flow;

FIG. 7 presents a further exemplary process flow; and

FIG. 8 presents another exemplary process flow.

DETAILED DESCRIPTION

Exemplary embodiments of a measurement arrangement and techniques fortesting an optical fiber, will now be described more fully withreference to the accompanying drawings. The measurement arrangement andtechniques may be especially suitable for testing and measurement ofoptical fibers when deployed in circumstances where relative movement ofa given anomaly in the optical fiber with respect to the optical fibertakes place during measurement. As used herein an anomaly may refer toany change in physical parameters induced in a fiber, including strain,temperature difference, and so forth. For example, it may be useful tomeasure a strain/temperature profile induced in an optical fiber, thenenvironmental temperature/strain profile, where relative motion takesplace between a sensing fiber of a measurement apparatus and theenvironmental temperature/strain profile during measurement. Thiscircumstance may obtain when an optical fiber, such as a fiber undertest, is being measured while deployed underseas, where at least aportion of the measurement apparatus is located on a ship or othervessel. Another circumstance is when a fiber is under test while beingcabled, where the fiber is in motion relative to the cabling apparatus.

Referring to FIG. 1A, there is shown a schematic diagram illustrating anexample of a measurement arrangement according to the prior art fortesting an optical fiber, in accordance with the present disclosure. Inthis embodiment, a measurement apparatus 101 is depicted with respect toa fiber under test (FUT), shown as optical fiber 110. The optical fiber110 may be arranged in any suitable form and location, for example, inan undersea fiber optic cable, or alternatively in a terrestrialsetting. The measurement apparatus 101 may include a probe source 102,to probe the optical fiber 110, such as a probe laser, arranged togenerate a probe beam 104 at a suitable wavelength for probing theoptical fiber 110. The measurement apparatus 101 may further include apulse modulator to receive first portion of a probe beam 114 from theprobe source 102, over a sensing fiber 103, and to output a plurality ofprobe pulses 108 to a fiber under test, shown as optical fiber 110.

As shown in FIG. 1A, the probe pulses 108 are generated at a frequencyν₀, and are characterized by a pulse width. The probe pulses, whenconducted to the fiber under test, optical fiber 110, may encounter ananomaly 112, such as a temperature/strain change, where the anomaly maygenerate Brillouin scattering of the probe pulses 108.

By way of background, when light enters an optical fiber photons may bescattered back toward the optical source, as well as forward. Brillouinscattered light is shifted in frequency from the original frequency ofthe probe beam toward lower frequency or higher frequency. Moreparticularly, Brillouin scattering may be generated by inelasticscattering of light in a physical medium by acoustical phonons with anaccompanying Brillouin Frequency Shift (BFS). Both temperature andstrain affect the medium density, then acoustic velocity ν_(a) and causechanges in the frequency of the Brillouin frequency shift ν_(BFS).

Techniques including Brillouin Optical Time Delay Reflection (BOTDR) andBrillouin Optical Time Delay Analysis (BOTDA) harness the measurement ofBrillouin scattered light to measure anomalies, such as localized strainchanges or localized temperature changes in an optical fiber, where thelocalized or distributed strain will affect the Brillouin frequencyshift. Generally, for Brillouin scattering, the change in BFS frequencycan be represented as Δν_(BFS)=C_(T)—ΔT+C_(ε)·Δε (1)

where C_(T) is approximately in the range of 0.75 MHz/C and C_(ε) is inthe range of 500 MHz/1% strain, where equation (1) forms the foundationof temperature/strain measurement based on detection of BrillouinScattering, for both BOTDR and BOTDA.

In accordance with embodiments of the disclosure, the measurementapparatus 101 may be used to perform Brillouin Optical Time DelayReflection as detailed below. For purposes of illustration, as shown inFIG. 1A, when a packet of 10³ photons are directed to the anomaly 112,generally one photon or less on average is scattered as Brillouinscattered light, meaning that the Brillouin scattering yield is lessthan or equal to 0.1 percent of the initial photons directed to theanomaly.

As depicted in FIG. 1A, the measurement apparatus 101 further includes aheterodyne receiver 114 that is arranged to receive a second portion ofthe probe beam 104 from the probe source (at frequency Do), and that isfurther arranged to receive a Brillouin scattered portion 115 of theprobe beam, scattered from the anomaly 112 of the fiber under test,optical fiber 110. As shown in FIG. 1A, this Brillouin scattered portion115 is received at a frequency ν₀+/−ν_(B), where ν_(B) is the Brillouinfrequency shift generated by the anomaly 112.

As further shown in FIG. 1A, the measurement apparatus 101 furtherincludes a digital processor 116, coupled to the heterodyne receiver114, and arranged to detect a position of the probe source with respectto the anomaly 112 while measuring the Brillouin scattered portion 115of probe beam 104. In accordance with various embodiments of thedisclosure, the probe source 102 and heterodyne receiver 114, as well asdigital processor 116 may be collocated with one another, where thedistance between probe source 102 and anomaly 112 may be approximatelythe same as the distance between heterodyne receiver 114 and anomaly112. Thus, the relative position of the heterodyne receiver 114 andanomaly 112 may be taken to be the relative distance between probesource 102 and anomaly 112.

According to embodiments of the disclosure, the apparatus 101 may beemployed to generate a Brillouin gain spectrum (BGS) based on probing onthe anomaly 112 in the fiber under test, optical fiber 110.

A Brillouin gain spectrum may generally comprise a plurality ofbackscatter traces, acquired at different instances. A given pulse ofthe plurality of pulses is launched at a frequency ν₀, as shown in FIG.1A, for example, where a given backscatter trace along the optical fiberis detected by the heterodyne receiver 114 at a frequency ν_(B), offsetfrom the frequency ν₀. Notably, the plurality of backscatter traces mayspan a predetermined frequency range that is characteristic of theBrillouin backscattering shift for a given fiber system, depending onthe properties of the anomaly, as shown above for equation (1).

More particularly, the measurement apparatus 101 may acquire a BGS alongan optical fiber in the following manner: For each launched pulse at ν₀,the back-scattered signal trace along the fiber at ν_(B) is detected bythe heterodyne receiver 114, while multiple probe pulses 108 arelaunched by the pulse modulator 106 to improve the signal-to-noise ratioof the BGS. The heterodyne receiver 114 may step the frequency through afull coverage of a predetermined frequency range to acquire the entireBGS. Notably, as discussed below with respect to FIGS. 4A and 4B, a BGSmay be presented as a three-dimensional graph plotting BGS intensity asa function of distance along a fiber on one axis and Brillouin frequencyshift along an orthogonal axis. Generally in a BGS, the spatialresolution is determined by the pulse width of the probe pulse, such asprobe pulse 108, while the total time for data acquisition of the entireBGS includes the scan duration for every frequency step.

Notably, in measurement scenarios where there is relative motion betweena sensing fiber and an anomaly such as a steady temperature and strainprofile in the fiber under test, assuming that the relative motion, V,<<Speed of light in fiber for simplicity, the spatial resolution will bedetermined not only by the pulse width of a launched pulse but also byV. The spatial resolution of a BGS may then be equal to the sum of thelimitation due to limited pulse width and the spatial distance of therelative motion during the duration between the start and end offrequency sweeping.

In one example of generating an adjusted BGS, the first trace of a BGSmay be collected for a first frequency step of −Δν, where the adjustmentis as follows: I₁₁(−Δν, x)→I′₁₁(−Δν, x)=I₁₁(−Δν, x). In this example,the intensity at −Δν, x is mapped to the same coordinates in a graphwhere distance (x) and frequency −Δν are parameters plotted in an “X-Y”plane and BGS intensity I is plotted along a Z-axis of a Cartesiangraph.

The N^(th) trace of the BGS may be collected for the same firstfrequency of −Δν, where the adjustment is as follows −Δν: I_(1N)(−Δν,x)→I′_(1N)(−Δν, x)=I_(1N)(−Δν, x−d_(1N)), where d_(1N) the distancemoved away along the direction of the pulse launch since the 1^(st)trace scan at the 1^(st) frequency step.

The N^(th) trace of the BGS may be collected for a different frequencyof −Δν+m×δν: I_(mN)(−Δν+m×δν, x as follows I′_(mN)(−Δν+m×δν,x)=I_(mN)(−Δν+m×δν, x−d_(mN)), where d_(mN) is the distance moved awayalong the direction of pulse launch since the 1^(st) trace scan at the1^(st) frequency step, and (−Δν, Δν) and is δν the frequency range andstep, respectively.

In the above manner, an initially unadjusted BGS may be adjusted andreconstructed as an adjusted BGS to account for movement of the anomalywith respect to the fiber as sensor during collection of the BGSspectrum. FIG. 2 illustrates a schematic showing components of ameasurement arrangement according to the present embodiments thatincorporates components of the arrangements of FIG. 1A or FIG. 1B forincorporating motion correction techniques during operation. As showntherein, the anomaly 112 may be represented as a moving profile of localtemperature/strain changes in the fiber under test. Said differently,the location or spatial distribution of the anomaly 112 may change inthe optical fiber 104 under test during collection of a BGS, resultingin a moving profile as shown. The measurement apparatus 101 may in turncollect this information (position d) in real time at the digitalprocessor 116, to account for the change in relative position of theanomaly 112 with respect to the optical fiber 104 under test, thusfacilitating the ability to generate an adjusted BGS.

Turning now to FIG. 1B there is shown a schematic diagram illustratinganother conventional measurement arrangement 150, according to the priorart, for testing an optical fiber, in accordance with the presentdisclosure. In this example, the measurement apparatus 151 sharessimilar components with measurement apparatus 101, with like componentslabeled the same. As such, operation of these components will not bediscussed in detail. The measurement apparatus 151 differs frommeasurement apparatus 101 in that a pump source 118 is provided, coupledto an opposite end of the optical fiber 110 as the probe source 102. Thepump source 118 may be a tunable CW laser, for example. As such, themeasurement apparatus 151 may be suitable to perform Brillouin OpticalTime Delay Analysis (BOTDA) for the fiber under test, where thescattering from the acoustic wave stimulated by the probe pulses 108 atanomaly 112 is amplified by counter-propagated CW pump light launchedfrom the other end of the fiber under test, e.g., generating StimulatedBrillouin Scattering (SBS). In such implementations, the BOTDAmeasurement approach generates a much better signal to noise ratio thanBOTDR, then longer reach, and better frequency and spatial resolution.During collection of a BGS using the measurement apparatus 151, therelative motion of the anomaly 112 with respect to the fiber 104 undertest may be recorded and taken into account to generate an adjusted BGS,generally as discussed above with respect to FIG. 1A. Notably, thearrangement of FIG. 1B requires access to both ends of a fiber undertest.

FIG. 3A illustrates a reference Brillouin gain spectrum resulting frommeasurement of a moving anomaly in an optical fiber. FIG. 3B illustratesa Brillouin gain spectrum resulting from measurement of the movinganomaly of FIG. 3A after correction for movement during the measurement,in accordance with embodiments of the disclosure. These spectra aresimulated spectra to illustrate operation of the present embodiments. InFIG. 3A, BGS intensity is plotted on the Z-axis, while Frequency anddistance are plotted on mutually orthogonal axes in the “X-Y” plane. Thevalues of frequency and position are arbitrary. As shown, the BGS isgenerated by a plurality of traces represented by the vertical planes,where the intensity increases to a peak that is centered about zerofrequency. Notably, the peak in BGS intensity at each successivefrequency step is shifted along the “position” axis with respect to theprior frequency step.

In FIG. 3B, a BGS is shown that is derived from the BGS of FIG. 3A,where the relative movement of the anomaly during BGS acquisition isaccounted for, such as described above. In this example, the BGS peakcaused by the anomaly does not shift in position with the differentfrequency steps.

FIG. 4A illustrates a Brillouin gain spectrum resulting from measurementof a moving anomaly in an optical fiber, where Brillouin scatteringintensity is plotted as a function of frequency and distance. FIG. 4Billustrates an adjusted Brillouin gain spectrum resulting fromcorrection of the Brillouin gain spectrum of FIG. 4A to account formovement of the anomaly during measurement, in accordance withembodiments of the disclosure. These spectra are experimentallymeasured. For better visibility the BGS is reversed. In FIG. 4A thereare actually two spectral peaks that are discernable. In FIG. 4B, asingle well defined peak is observed, where the distance is shifted 100m or so than is apparent in FIG. 4A. Thus the position of the anomaly ismore accurately and precisely determined.

FIG. 5 presents an exemplary process flow 500, in accordance withembodiments of the disclosure. At block 502 a plurality of probe pulsesare launched from a probe source, such as a laser source. The pluralityof probe pulses may be output by a pulse modulator, coupled to the lasersource. The plurality of probe pulses may be output over a given timeinterval and may be directed to an anomaly of an optical fiber thatrepresents a fiber under test.

At block 504 a Brillouin gain spectrum is recorded from a plurality ofreflection signals generated in the optical fiber, responsive to theplurality of probe pulses. In some examples, a group of reflectionsignals may be collected by a heterodyne receiver for each frequencystep of a series of frequency steps centered around a characteristicfrequency, representing the Brillouin frequency shift generated by ananomaly in the fiber under test. In various embodiments, the anomaly maybe characterized as a local change in strain/temperature in the opticalfiber, where a change in Brillouin frequency Δν_(BFS) is generatedaccording to Δν_(BFS)=C_(T)·ΔT+C_(ε)·Δε, where C_(T) represents thefrequency shift per degree Celsius change in temperature, and C_(ε)· isthe change in frequency per % strain in the optical fiber. In somenon-limiting embodiments, the value of C_(T)· is in the range of 0.75MHz/C and the value of C_(ε)˜500 MHz/1% strain.

At block 506, the relative motion between the anomaly in the fiber undertest and the probe source or heterodyne receiver may be determinedduring the recording of the Brillouin gain spectrum.

At block 508, the Brillouin scattering spectrum dynamically adjustingaccording to the relative motion between probe source and anomaly. Inone example of generating an adjusted BGS, from a first trace to a lasttrace of a BGS may be collected where the N^(th) trace of the BGS may becollected for a different frequency of −Δν+m×δν: I_(mN)(−Δν+m×δν, x asfollows I′_(mN)(−Δν+m×δν, x)=I_(mN)(−Δν+m×δν, x−d_(mN)), where d_(mN) isthe distance moved away along the direction of pulse launch since the1^(st) trace scan at the 1^(st) frequency step, and (−Δν, Δν) and is δνthe frequency range and step, respectively.

FIG. 6 presents another exemplary process flow 600. At block 602 arelative motion is measured between a probe source an anomaly in anoptical fiber under test. The probe source may be a laser source thatlaunches a series of probe pulses that are directed to the anomaly. Theanomaly may be a local temperature/strain profile in the optical fiberunder test. At block 604, a start of acquisition of a Brillouin gainspectrum (BGS) is synchronized with the detection of motion of thetemperature/strain profile. The BGS may constitute a plurality ofbackscatter traces, that acquired at a plurality of instances.

At block 602, after completing the of acquisition of the BGS, the BGS iscorrected based on a position of the anomaly at a time when a given BGStrace of the BGS is acquired.

FIG. 7 presents a further exemplary process flow, shown as flow 700. Atblock 702 one or more pulses is launched at each wavelength into sensingfiber with a pulse rate no larger than 1/round trip time. At block 704synchronized real time acquisitions of back-scattering light power andrelative motion/position between sensing fiber and spectral profile ofphysical parameters, such as BGS, under test are performed. At block 706the operation is performed to convert real time acquisition of eachback-scattering power data point or each trace from time-space todistance-space with instant correction. At block 708 the operation isperformed to align spatially distributed traces for the same wavelengthbased on relative position readings before averaging or otherstatistical analysis. At block 710, the operation is performed to alignspatially distributed traces from step above for all wavelengths basedon relative position readings to construct the actual spatiallydistributed spectral profile such as BGS under test with the skewingfrom relative motion corrected.

After block 704, the flow may proceed to block 712 to step to the nextwavelength. After block 708, the flow may proceed to block 714 to stepto the next wavelength. After block 704, the flow may proceed to block716 where the operation is performed to convert real time acquiredback-scattering power data points from time-space to distance-space andmake proper data alignments. For example, the data set for each tracemay be converted with relative motion correction, the operation may beperformed to align and then statistically process spatially distributedtraces for each wavelength, and the operation may be performed to alignprocessed spatially distributed traces for all wavelengths to constructspatially distributed spectral profile with correction based on relativemotion/position.

FIG. 8 presents another exemplary process flow, shown as process flow800. At block 802 one or more pulses is launched at each wavelength intosensing fiber with a pulse rate no larger than 1/round trip time. Atblock 804 each pulse triggers a sequence of real time acquisitions ofback-scattering light power data points and one acquisition of relativemotion/position between sensing fiber and profile of physical parametersunder test. At block 806 real time acquisition of each trace isconverted from time-space to distance-space without correction of motionskewing between data points. At block 808 align spatially distributedtraces are aligned based on the relative position readings at each pulsebefore averaging or other statistical analysis is performed. At block810 spatially distributed traces are aligned from the step above for allwavelengths based on relative position readings to construct the actualspatially distributed spectral profile such as BGS under test with skewsbetween traces and/or between wavelengths from relative motioncorrected.

As used herein, an element or step recited in the singular and proceededwith the word “a” or “an” should be understood as not excluding pluralelements or steps, unless such exclusion is explicitly recited.Furthermore, references to “one embodiment” of the present disclosureare not intended to be interpreted as excluding the existence ofadditional embodiments that also incorporate the recited features.

While the present disclosure makes reference to certain embodiments,numerous modifications, alterations and changes to the describedembodiments are possible without departing from the sphere and scope ofthe present disclosure, as defined in the appended claim(s).

For example, the aforementioned techniques may be applied to any fiberoptical distributed sensing of spectral profile of physical parameters,such as BGS with BOTDR and Raman spectrum with Raman-OTDR. Accordingly,it is intended that the present disclosure not be limited to thedescribed embodiments, but that it has the full scope defined by thelanguage of the following claims, and equivalents thereof.

The invention claimed is:
 1. A method of measuring an anomaly in anoptical fiber, comprising: launching a plurality of probe pulses from aprobe source into the optical fiber; recording a Brillouinback-scattering spectrum from a plurality of reflection signalsgenerated in the optical fiber, responsive to the plurality of probepulses; determining a relative motion between the probe source and theanomaly during the recording of the Brillouin back-scattering spectrum;and dynamically adjusting the Brillouin back-scattering spectrumaccording to the relative motion.
 2. The method of claim 1, wherein therecording the Brillouin back-scattering spectrum comprises performingBrillouin Optical Time Domain Reflectometry (BOTDR) or comprisesperforming Brillouin Optical Time Domain Analysis (BOTDA).
 3. The methodof claim 1, wherein the Brillouin back-scattering spectrum comprises aBrillouin gain spectrum.
 4. The method of claim 3, wherein the probesource comprises a sensing fiber, wherein the anomaly comprises a changein temperature and/or strain (temperature/strain) in the optical fiber,and wherein measurement of a relative motion of the sensing fiber and atemperature/strain profile is synchronized with detection of theBrillouin gain spectrum.
 5. The method of claim 4, wherein the Brillouingain spectrum comprises a plurality of backscatter traces, acquired at aplurality of instances, wherein after an initial backscatter trace ofthe plurality of backscatter traces, subsequent traces of the pluralityof traces are shifted forward in space when the anomaly is moving awayfrom the probe source, and wherein after an initial backscatter trace ofthe plurality of backscatter traces, subsequent backscatter traces ofthe plurality of backscatter traces are shifted backward in space whenthe anomaly is moving toward the probe source at one end of the opticalfiber.
 6. The method of claim 5, wherein a given pulse of the pluralityof pulses is launched at a frequency ν₀, wherein a given backscattertrace along the optical fiber is detected by a heterodyne receiver at afrequency ν_(B), and wherein the plurality of backscatter traces span apredetermined frequency range.
 7. The method of claim 6, wherein a givenBrillouin gain spectrum is characterized for a first trace at a firstfrequency step as −ΔνI₁₁(−Δν, x)→I′₁₁(−Δν, x)=I₁₁(−Δν, x), and for anN^(th) trace at as Δν: I_(1N)(−Δν, x) →I′_(1N)(−Δν, x)=I_(1N)(−Δν,x−d_(1N)), wherein d_(1N) a distance moved away along the direction of apulse launch since the 1^(st) trace at the 1^(st) frequency step.
 8. Themethod of claim 7, wherein a given Brillouin gain spectrum ischaracterized for a N^(th) trace at −Δν+m×δν: I_(mN)(−Δν+m×δν,x)→I′_(mN)(−Δν+m×δν, x)=I_(mN)(−Δν+m×δν, x−d_(mN)), where d_(mN) is adistance moved away along the direction of pulse launch since the 1^(st)trace scan at the 1^(st) frequency step, (−Δν, Δν) and δν being thefrequency range and step, respectively.
 9. A method of measuring ananomaly in an optical fiber, comprising: measuring a relative motionbetween a probe source and the anomaly; synchronizing a start of anacquisition of a Brillouin gain spectrum (BGS) and an anomaly motiondetection, wherein the BGS comprises a plurality of backscatter traces,acquired at a plurality of instances; and after completing of theacquisition of the BGS, correcting the BGS based on a position of theanomaly at a time when a given BGS trace of the plurality of BGS tracesis acquired.
 10. The method of claim 9, wherein the BGS is acquired byBrillouin Optical Time Domain Reflectometry (BOTDR) or is acquired byBrillouin Optical Time Domain Analysis (BOTDA).
 11. The method of claim9, wherein the probe source comprises a sensing fiber, wherein theanomaly comprises a change in temperature and/or strain(temperature/strain) in the optical fiber, and wherein measurement of arelative motion of the sensing fiber and a temperature/strain profile issynchronized with detection of the Brillouin gain spectrum.
 12. Themethod of claim 11, wherein after an initial backscatter trace of theplurality of traces, subsequent backscatter traces of the plurality ofbackscatter traces are shifted forward in space when the anomaly ismoving away from the probe source, wherein after the initial backscattertrace of the plurality of backscatter traces, subsequent backscattertraces of the plurality of traces are shifted backward in space when theanomaly is moving toward the probe source.
 13. The method of claim 9,wherein a given pulse of the plurality of pulses is launched at afrequency ν₀, wherein a given backscatter trace along the optical fiberis detected by a heterodyne receiver at a frequency ν_(B), and whereinthe plurality of backscatter traces span a predetermined frequencyrange.
 14. The method of claim 13, wherein a given Brillouin gainspectrum is characterized for a first trace at a first frequency step as−ΔνI₁₁(−Δν, x)→I′₁₁(−Δν, x)=I₁₁(−Δν, x), and for an N^(th) trace at as−Δν: I_(1N)(−Δν, x)→I′_(1N)(−Δν, x)=I_(1N)(−Δν, x−d_(1N)), whereind_(1N) a distance moved away along the direction of a pulse launch sincethe 1^(st) trace at the 1^(st) frequency step.
 15. The method of claim14, wherein a given Brillouin gain spectrum is characterized for aN^(th) trace at −Δν+m×δν: I_(mN)(−Δν+m×δν, x)→I′_(mN)(−Δν+m×δν,x)=I_(mN)(−Δν+m×δν, x−d_(mN)), where d_(mN) is a distance moved awayalong the direction of pulse launch since the 1^(st) trace scan at the1^(st) frequency step, (−Δν, Δν) and δν being the frequency range andstep, respectively.